This invention relates to fluororubber compositions and a method for preparing the same, and more particularly, to peroxide-crosslinking fluororubber compositions having improved mechanical strength as well as good solvent resistance, chemical resistance, heat resistance, and low-temperature properties.
Prior art vinylidene fluoride base fluororubbers are used in a variety of applications mainly in the automobile and machinery industries since they are elastomers having good heat resistance and mechanical strength.
These fluororubbers, however, are not satisfactory in chemical resistance. They are readily swollen in polar solvents such as ketone, lower alcohol, carbonyl and organic acid solvents. They are deteriorated by amine-containing chemicals so that their rubber strength and elongation may substantially decline.
In order to eliminate these drawbacks, curable fluorine-containing compositions comprising a perfluoro compound and a fluorinated organohydrogenpolysiloxane as main components were proposed. These compositions, however, are liquid because they are based on liquid perfluoro compounds having a low degree of polymerization. Then the compositions are suitable in the formed in-place gasket (FIPG) process and liquid injection molding system (LIMS), but less effective in compression molding commonly employed for rubber molding.
In particular, the curable fluorine-containing compositions often prohibit the use of conventional two-plate molds for rubber because of the difficulty of molding and the frequent occurrence of defects by air entrapment. Steady production is difficult unless a special LIMS mold is newly fabricated. The LIMS mold, however, has the problems that it is generally more expensive than the conventional two-plate molds for rubber, the mounting of the mold in a LIMS molding machine is cumbersome, and the molding machine requires time-consuming adjustment after mold mounting. The LIMS mold does not lend itself to the manufacture of a variety of parts in small quantities.
Under the circumstances, the applicant/assignee previously proposed in JP-A 2001-164115 (corresponding to U.S. Ser. No. 09/729,774 or U.S. 2001-0008914A) a method for preparing a rubber composition of the type that can be roll milled and molded in a compression molding rubber mold, referred to as millable composition, from a fluorinated curable composition comprising a perfluoro compound and a fluorinated organohydropolysiloxane as main components.
Although the curable composition of our previous proposal exhibits satisfactory physical properties when addition crosslinked, the addition crosslinking suffers from a crosslink-inhibitory problem that the catalyst is deactivated by compounds containing N, S, P or similar atoms, commonly known as catalyst poisons.
In actual rubber molding locations, compounds containing N, S, P or similar atoms are commonly used as the crosslinking agent or antioxidant for synthetic rubber. These compounds deposited on twin rolls or mold can become catalyst poisons to give rise to the crosslink-inhibitory problem.
A switch to peroxide crosslinking can avoid the catalyst poison. It was found that if the composition of the above proposal is converted to a peroxide crosslinking system, the physical properties, especially strength, are degraded as compared with the addition crosslinking.
Therefore, an object of the invention is to provide a peroxide crosslinking fluororubber composition having improved mechanical strength as well as good solvent resistance, chemical resistance, heat resistance, and low-temperature properties, and a method for preparing the same.
It has been found that a fluororubber composition comprising a precured base obtained by combining (A) a liquid perfluoro compound and (B) a compound capable of addition reaction having at least two hydrosilyl groups and precuring them in the presence of an addition reaction catalyst, (C) a reinforcing filler, and (D) a peroxide crosslinking agent containing an isopropyl monocarbonate group exhibits good heat resistance, solvent resistance, chemical resistance, and low-temperature properties and experiences a minimal decline of physical properties, especially strength, despite peroxide crosslinking.
Accordingly, the present invention provides a fluororubber composition comprising
(A) a liquid perfluoro compound having at least two alkenyl groups in the molecule and a divalent perfluorolakylene or perfluoropolyether structure in the backbone,
(B) a compound capable of addition reaction having at least two hydrosilyl groups in the molecule,
components (A) and (B) being combined in such amounts as to give a molar ratio of hydrosilyl groups in component (B) to alkenyl groups in component (A) of 0.1/1 to 0.99/1 and precured in the presence of an addition reaction catalyst to form a precured base,
(C) 1 to 100 parts by weight per 100 parts by weight of component (A) of a reinforcing filler, and
(D) 0.1 to 10 parts by weight per 100 parts by weight of component (A) of a peroxide crosslinking agent containing an isopropyl monocarbonate group in the molecule.
The fluororubber composition is prepared through the steps of combining components (A) to (C) for reaction and then adding and mixing component (D) with the resulting mixture.